Parallel filoter while intuitively a good thought rarely work on a boat
in practice. A high particle challenge situation that 'slug' one
filter will sequentially 'slug the second filter. It is VASTLY more
efficient to run a recirculation / polishing system INDEPENDENT of the
in line fuel deliver line as a high turn over recirculation filter will
VASTLY 'turn the tank over' much quicker )3-4 gallones per minute) than
an in-line filter (1 gallon per hour average). The advantage being
FASTER tank recovery back to 'normal' particle background .... using a
filter retention of approx 10 times the retention of the final filter
in the direct feed system. A 'techy' would call this the laws of
exponential decay PLUS the 10X retention filter does have an
efficiency (although low) at much smaller than the 'rating'. Since you
'turn-over' so much faster though the larger retention filter (with 10X
lower flow resistance) the net result is VERY fast clean up of the tank
contents.

Send me an email at RhmpL33(AT)att(DOT)net for a sketch of my
filtration train ( independent recirc. plus blocked and bypassed dual
parallel direct line filters .... plus on-line reserve (emergency) day
tank.
The day tank is a gravity feed 3 gallon 'storage' medium for clean fuel
.... so that if all hell breaks loose I can simply switch it IN to the
direct feed line and run for several hours .... and dont have to worry
about changing the diaphragm in the lift pump, could care less about
changing racors, etc. etc. etc. until 'things settle down'. I dont
know about you but usually a heavy sea state will knock crap loose in a
tank ... and thats NOT the time to go below and screw around with
'filters' - when the sea is running hard, if I put my head into the
bilge for any length of time ... I power -puke.